This invention relates to a buffer circuit of the kind incorporating a metal oxide-silicon field-effect transistor (MOSFET) for use in buffering circuits and sensors such as photosensitive elements and other transducers.
Buffer circuits are well known for the purposes of isolating sources of weak signals, sensitive circuitry or components, particularly (although not exclusively) in the field of sensors such as radiation detectors. Use of MOSFETs in buffer circuits is disclosed in U.S. Pat. No 4,808,822 to Manning and Watton, which relates to a thermal detector comprising one or more rows of pyroelectric elements. The elements develop output signals in response to modulated radiation from a thermal scene. The or each row has a respective common row output line for output signals from all elements in the row. Each element is buffered from the output line by a respective power amplifier in the form of a MOSFET source follower; this avoids loss of signal arising from the much greater capacitance of the row output line (xcx9c30 pf) compared to that of each element (xcx9c1 pF), which would otherwise have a potential divider effect and reduce the output signal by more than an order of magnitude.
The buffer circuit of U.S. Pat. No. 4,808,822 suffers from the disadvantage that there may be variation between the properties of individual MOSFETs (eg threshold voltages) which would introduce false contrast into the image. More importantly, it has no effect on a fundamental problem in pyroelectric detector arrays, an unwanted signal component known as fixed pattern noise. This arises from the differing thermal response characteristics of the detector elements; if the elements were perfectly matched, exposing them to a constant temperature background should result in output signals which are equal to one another, but this is very far from the case; instead there is a variation between element output signals which is very much larger than that of image contrast associated with a typical ambient thermal scene. To deal with this it is necessary to store successive image frames associated respectively with dark field (scene obscured) and light field (scene unobscured) conditions, and to subtract the former from the latter. This must be done very acurately to obtain the required scene contrast component, which is small and can be lost in subtraction error.
A similar problem occurs with radiation detetors of semiconductor material, such as for example that described in U.S. Pat. No. 5,155,348 to Ballingall and Blenkinsop. This patent relates to a readout circuit for a photodiode; the circuit stores a photodiode output signal produced in a calibration phase for later use in subtraction from another output signal obtained in a measurement phase. Here again the purpose is to remove a very large unwanted signal component as soon as possible in processing to avoid burdening later circuitry with requirements for processing capacity and accuracy. The scale of the problem is illustrated in the field of photoconductive detectors, where the bias voltage on such a device in operation is in the region of 1 volt, radiation from an ambient thermal scene at 290 K gives a signal in the order of 1 millivolt, and scene contrast (ie the required image information) is a few microvolts. The problem of detecting small signals in the presence of very large offsets has been appreciated for many years, and reducing the processing circuitry needed to achieve this is a long-felt want.
It is an object of the present invention to provide an alternative form of buffer circuit suitable for programming to counteract offsets, mismatches and the like.
The present invention provides a buffer circuit including a MOSFET power amplifier buffer (eg a source follower) and a second MOSFET arranged to control current through an input or output node of the circuit, characterized in that the second MOSFET has a floating gate on which charge is storable to change circuit properties and the circuit has means for storing charge on the floating gate.
Floating-gate MOSFETs are known, as described for example in U.S. Pat. No. 5,557,234 to Collins, and generally as outlined by Sze in xe2x80x9cPhysics of Semiconductor Devicesxe2x80x9d, 2nd Ed. Wily 1981, page 496. In IEEE Electron Device Letters Vol 12, No. 3, March 1991, Thomsen et al estimate that a floating gate in a silicon MOSFET would lose its charge at the rate of 0.1% in 26 years, which means that for practical purposes charge is retained undiminished on the gate permanently unless reprogrammed.
The invention provides the advantage that storing charge on the floating gate alters the threshold voltage of the MOSFET and hence also its channel conductivity, enabling the current through an input or output circuit node to be preset for a predetermined input signal; it provides a mechanism for altering circuit properties if required to trim either a single circuit or an array of like circuits to obtain required or matched operating characteristics. Alternatively, it may be used to counteract an unwanted contribution to the circuit input signal.
There are a number of techniques for charging or programming the floating gate of the second MOSFET. One technique is described in U.S. Pat. No. 5,557,234 for a MOSFET having a window through which ultraviolet (UV) light can be applied to an insulation layer between the MOSFET floating gate and its control gate. When a voltage is placed on the control gate, the insulation layer is UV illuminated and becomes conducting allowing charge to be transferred from the control gate to the floating gate. Hot electron injection or Fowler-Nordheim tunnelling may also be employed for charging the floating gate.
The second MOSFET may be connected in series with a sensor itself connected to the input node, the second MOSFET being used to counteract unwanted sensor characteristics. The circuit may be part of an array of like circuit associated with respective sensors and providing means for counteracting differences between the characteristics of the sensors or of the circuits themselves. The sensors may be (but are not limited to) radiation sensors such as pyroelectic elements, photoconductors, phototransitors or photodiodes. Each circuit may be trimmable by storage of charge on its respective floating gate to conform to a common output criterion.
The second MOSFET may be connected in series with a sensor and arranged to control current through the latter. It may have a control gate and a drain which are connected together to provide capacitative coupling between the drain and floating gate via the control gate. The MOSFET power amplifier may be a source follower and the sensor a phototransistor with an emitter connected to the second MOSFET drain, which in may be connected to a gate of the source follower, which may be in series with a switch (M55) actuatable to relay source follower output signals to an output.
The circuit may be switchable between on and off states, the source follower being switched on only when read-out is required.
In one embodiment, the second MOSFET has a control gate and the buffer circuit is:
a) a member of an array of like circuit,
b) selectable to store charge on the floating gate during a programming operation, and
c) deselectable to avoid storage of charge on the floating gate during a programming operation associated with another circuit of the array.
In a further embodiment, the MOSFET power amplifier is a source follower with a gate connected to receive signals from a phototransistor in series with a load comprising at least one diode-connected MOSFET, the second MOSFET is arranged to control current though an output circuit node comprising a source of the source follower and is in series with the source follower and a switch, and the switch is actuatable to select the circuit and relay output signals from it to a circuit output.
Alternatively the second MOSFET may be connected as a load of the MOSFET power amplifier arranged as a source follower, and may be arranged in series with a switch to control current through an output circuit node comprising a source of the source follower, and the switch and the second MOSFET being actuatable jointly to select the circuit and provide for output signals from it to pass to a circuit output.
The buffer circuit of the invention may a member of an array of like source follower circuit which are trimmable by storage of charge on respective floating gates and the means for storing charge comprises programming means arranged both to store charge on the respective second MOSFET floating gate in response to programming intended for that circuit and to densensitise it to programming intended for another circuit.
In another aspect, the invention provides a method of trimming buffer circuit characterized in that it comprises the steps of:
a) providing a buffer circuit incorporating a MOSFET power amplifier buffer and a second MOSFET arranged to control current through an input or output node of the circuit, the second MOSFET having a floating gate on which charge is storable to change circuit properties;
b) adjusting the charged stored on the floating gate coarsely by at least one of hot electron injection, Fowler-Nordheim tunnelling and ultraviolet illumination until the circuit provides an output approximating to a required output;
c) adjusting the charge stored on the floating gate finely by application of less than one hundred pulses of more than 1 second""s duration to an injector or control gate of the second MOSFET.
The method of the invention is applicable when the circuit is a member of an array of like circuits, and in this case it may include the step of desensitising circuit to adjustment when not required by applying to a control gate of the second MOSFET a deselect voltage greater in magnitude than a supply voltage of the circuit.
The method of the invention is also applicable when the circuit is a pixel circuit for an imaging system, and in this case adjustment in step (c) may be carried out when the pixel is illuminated with radiation corresponding to the middle of its dynamic range on a logarithmic scale, or alternatively with radiation corresponding to a low ambient light level.
In an alterative aspect, the invention provides a method of trimming a buffer circuit characterized in that it comprises the steps of:
a) providing a buffer circuit incorporating a MOSFET power amplifier buffer and a second MOSFET arranged to control current through an input or output node of the circuit, the second MOSFET including a floating gate on which charge is storable to charge circuit properties; and
b) adjusting the charge stored on the floating gate until the circuit has an output voltage which is differs from a required output voltage by an error voltage xcex4V, where:             δ      ⁢              xe2x80x83            ⁢      V        =                            C          inj                ⁢                  V          inj                            C        TOT              ,
Cinj is the capacitance between the floating gate and a second MOSFET component employed to store charge on the floating gate (F67), Vinj is the voltage on that component and CTOT is the total floating-gate capacitance.